I am using Photoshop
5+, but I imagine similar methods are available in other software packages.

In this tutorial
I will make a background star field with various star sizes. I will then
explain methods I use to make nebulous clouds. Lastly I will explain how
to use brushes to make additional stars and starbursts.

To create
a general star background, I use basic filters. Start with a new image
and make it black. I started an image using resolution of 720x480 so some
settings I use may need adjustment if you use different resolution.

Add
noise to the image. In Photoshop, I use the Noise Filter at about
20%, Gaussian, Monochromatic. I've found good results with noise
between 5% and 40%. This image is Noise as it appears in Photoshop
6.

Adjust the
Brightness and Contrast of the Star Background layer to bring out some
stars. I adjusted the Brightness to around -90, and the contrast to about
+40. Good results can be achieved by applying Brightness and Contrast
of different values multiple times.

Now duplicate
this layer. Select the duplicate layer, and adjust the scale by selecting
Edit>Transform>Scale. Increase the scale of the overlapping
image to somewhere between 200% and 500%. Now change the layer blending
mode to screen.

The additional layer will add larger stars
to the image.

Now we will
create some interesting nebulous clouds using one of my favorite filters,
Clouds. First decide on the main color of the nebulous clouds.
In this case I chose a light blue. Be sure the background color is black.
On a new layer render clouds by clicking Filter>Render>Clouds.

Render
Clouds,
main color foreground,
background color black.

Now I create
a more interesting and defined shape by using difference clouds. Do this
by clicking Filter>Render>Difference Clouds. I apply this
multiple times (Ctrl-F to repeat last filter) until the clouds shape become
more vein like.

Difference
Clouds 2+ times.

Essentially,
we want the darker area of the cloud layer to be transparent. To do this
we change the blending option to Screen. But this isn't enough, we want
to adjust the blending using the advanced blending options. By adjusting
the slider
icon on "This Layer", we can adjust the range of transparency,
effectively controlling the range that darker pixels are dropped from
the layer. Additionally, by holding the Alt key and clicking and dragging
on one half of the slider icon, we can adjust the range of partially blended
pixels.

I adjusted
the transparency on "This Layer" by adjusting the darker, left
hand side, sliders to 25 and 145 as shown in the image below. The resulting
image shows the breakthrough of the star layer below the cloud layer.
Adjust blending to your liking.

Now to add
some color add an additional cloud layer above the other layers. Choose
a color that will compliment your main cloud color. Apply render clouds
and difference clouds similarly to that as for the main cloud layer. /p>

Adjust the
compliment cloud layer similar to the main cloud layer. But change Blending
type to Hue, and adjust the "This Layer" blending. Your main
cloud layer become more interesting. Adjust the compliment cloud layer
to you liking.

Sometimes,
I like to link the Main Cloud (Cloud Screen) and Compliment Cloud (Cloud
Hue) layers together and zoom into a focus area using the transform scale
option. In this case, I focused on a particularly interesting area of
the clouds. I used the Free Transform option to zoom and place the cloud
layer how I wanted. Free Transform can be applied by hitting Ctrl-T or
by clicking Edit>Free Transform.

Note: because
of the transform scales, the file size and memory requirements for the
file will become larger. To improve performance and reduce file size you
may want to remove the parts of the image that exist outside the viewable
Canvas. To do this we will crop the image. First choose Select>All
(or hit Ctrl-A) and then select Image>Crop.

Now add an
additional layer. On this layer we will use various brushes make additional
stars. Initially, I use the smaller brushes and set color to white or
yellow (star color is up to you). On the new layer, set the brush fade
to 1, and start adding stars using various soft round brushes.

Now we will create
some custom brushes to add starbursts to some. Start a new image and set
its size to 1 in by 1 in.

On the Background
layer, draw a thin plus sign. Add an additional layer and use the Radial
Gradient fill (or use a soft circular brush if you'd rather) to make a
blurred dot. Set the blurred dot layer to Multiply.

background

+

multiply

=

Merge the
layers by clicking Layer>Merge Visible. I recommend blurring
the starburst image. I used the Blur More filter twice.

Now select
the entire 1in by 1in area (Ctrl-A) and define it as a new brush by selecting
Edit>Define Brush. I repeated this process multiple times, each
time I reduced the image size by 12 pixels until I had five new starburst
brushes.

Now use the brush tool with these new bruses and daub them over the stars
already in the image. That is it. Now that you understand the process,
you should be able to create interesting starscapes in no time.